Armed For Alfalfa Weevils

Chuck Noble doesn't see much alfalfa weevil damage in his 300 acres of alfalfa near Winner, SD. But that doesn't mean the weevils aren't lurking. It means Noble is a success at applying insecticide early enough to kill the egg-laying beetles before they drop their eggs. There's a tendency for people to put their heads in the sand and not do much about alfalfa weevils around here, Noble says. They

Chuck Noble doesn't see much alfalfa weevil damage in his 300 acres of alfalfa near Winner, SD. But that doesn't mean the weevils aren't lurking.

It means Noble is a success at applying insecticide early enough to kill the egg-laying beetles before they drop their eggs.

“There's a tendency for people to put their heads in the sand and not do much about alfalfa weevils around here,” Noble says. “They let crops get devastated and wait for a second crop in the extreme. At three to six weevils/plant, they can consume one-half to one ton of alfalfa/acre.”

Noble's fields are scouted in late April, “as soon as the beetles come out of dormancy.” When the temperature gets above 50-55° and he sees a threshold of one beetle per square yard, he has his fields sprayed.

“One beetle lays 500 eggs or more. You don't need many more than that to have a problem,” he says. “I'm not in the practice of letting these things go, so I haven't measured what a totally devastated, chewed-up field is. But I'm sure you're looking at an RFV of 50 or 100.”

Noble's fields average 160-190 RFV, primarily because they're sprayed before egg-lay, he says.

“It only costs me $6/acre to spray because I apply it with fertilizer. If I didn't do it early, I'd probably have to spend $12/acre because I don't fertilize when the crop is 12" to 18" to 20" tall. If my crop is being chewed, at a minimum I've lost 25-30 RFV points, which, in rough terms, you could say would be a half dollar/RFV point. That's more than $12/ton right there, plus lost tonnage.”

Spraying before alfalfa weevil egg-lay wouldn't work in Oklahoma because of its different climate, says Richard Berberet, Oklahoma State University entomologist.

“The difference between us and the North is that they pretty well know when the weevil is going to show up,” he adds. “Most of their egg-laying takes place in spring. We have egg-lay for six months, say from November until April or May.

“In areas where the egg-lay is fairly concentrated in a shorter time period, you may be able to spray out beetles before the egg-lay has really gotten going. For us, it's such a protracted thing — and we get so much beetle movement throughout the winter that attempting to spray for adults doesn't help us much.”

So what do Oklahoma growers do?

“A majority of our farmers turn cattle out on alfalfa stands during winter to graze them very cleanly,” says Berberet. “The cattle eat alfalfa weevil eggs as they eat the green crown growth and the browned-off growth from the previous fall. We can reduce our egg populations by probably 50% with grazing.”

That, in turn, can delay the need for first insecticide treatments by two to three weeks. “A lot of our good managers can control weevils pretty well with winter grazing and a single, well-timed insecticide application. Then, close to first cutting, we let the parasitic wasps mop up the remaining larvae.”

Yet parasitics — used successfully in states like Minnesota — haven't provided much control in Southern Great Plains states like Oklahoma. Although three parasitic wasp species have been established in Oklahoma, “each encounters unfavorable climatic conditions here they don't encounter in the North,” he says.

One problem is that parasites normally cocoon on the soil surface in alfalfa fields during summer.

“It's not unusual to have 20-30 days straight of 100° heat and soil temperatures at 120-130°,” he says. “In some cases, 90% of the parasites are killed within their cocoons due to heat and desiccation in summer. In Minnesota or Michigan or New York, they don't have to contend with that.

“Alfalfa weevils take a different approach; their adult beetles leave fields to avoid summer heat. They hide in fencerows and creek bottoms and under vegetation. The pest has the behavior that allows it to survive the extremes. The parasite doesn't.”

Berberet and colleagues begin surveying alfalfa weevil egg populations in January and again in mid-February. With numbers up to 200/square foot, growers can expect a one-spray year; counts often exceed 500, which means multiple applications may be needed, he says.

“We run our predictions for damaging larval populations entirely on a degree day calendar. Once we get 150 degree days accrued (above 50°), we start scouting for larvae.”

He mentions the Oklahoma Mesonet (www.mesonet.ou.edu) as one tool Oklahoma growers can use to check when the 150 degree days are reached in any Oklahoma county.

“In many areas around the country, there's a question of whether you would even have a sprayable weevil population. We don't have that question. We know the alfalfa weevil larvae will be here in abundance; it's just a matter of how high the population levels will be and when sprays should be applied.”

Noble is willing to discuss his method of weevil treatment and can be contacted at 425-747-7092.